The present invention relates to a level sensor which is developed so as to determine the undershooting or overshooting of a predetermined level of liquid in a container, the sensor having a resistance element which is traversed by electric current and changes its electrical resistance upon an increase in the temperature, and having a measuring unit for measuring the electric resistance of the electrical conductor. The invention also relates to a method of determining the undershooting or overshooting of an intended level of liquid in a container, in which method a resistance element which is arranged at the intended level of filling and changes its electrical resistance with an increase in temperature is heated by an electric current whereupon its electrical resistance is measured.
Such limit-level sensors are frequently used in motor vehicles, for instance in a transmission, an internal combustion engine or a fuel tank and are known from actual practice. As resistance element, the limit-level sensor has a resistance wire which extends down into the region of the bottom of a container to be measured. For the measurement, the resistance wire is traversed by an electric current. In this way, the resistance wire is heated, thereby changing its electrical resistance. Since the heat produced thereby in the region of the resistance wire immersed in the liquid is substantially more strongly led away from the resistance wire in the liquid than the region above the liquid, the level of the liquid can be calculated from the resistance ascertained and compared with a minimum level.
This method has the disadvantage that the resistance of the resistance wire which is determined is influenced by the temperature of the liquid. The determination of the level is thus relatively inaccurate. Therefore, a measurement of the minimum level of motor oil in an internal combustion engine of a motor vehicle having this level sensor is possible only at great expense.
For the dependable determination of the level of a liquid having a particularly high temperature, floats provided with a magnet which float on the liquid are known. When an intended level is undershot or overshot, the magnet switches a reed contact which is arranged at the height corresponding to the level. Such a limit-level switch, however, is particularly expensive. Furthermore, this level switch has movable parts which can jam.